1. Field of the Invention
The present invention relates to a device for detecting an oxygen concentration, and more particularly to a device for detecting an oxygen concentration in a gaseous body such as exhaust gas of an internal combustion engine.
2. Description of Background Information
For the air/fuel ratio control of an internal combustion engine, air/fuel ratio feedback control systems are generally utilized to assure the purification of exhaust gas and improvements of the fuel economy. In those systems, oxygen concentration in exhaust gas is detected, and an air/fuel ratio of the mixture to be supplied to the engine is feedback controlled toward a target air/fuel ratio in response to a result of the detection of oxygen concentration.
As an oxygen concentration detection device for use in such an air/fuel ratio control system, there are devices of a type capable of producing an output signal whose level is proportional to the oxygen concentration in the object gas. As an example, Japanese Patent Application Laid Open No. 52-72286 discloses an oxygen concentration detection device which includes oxygen-ion conductive solid electrolyte members each having a pair of electrodes thereon. The surface of one of the electrodes of the solid electrolyte member forms a part of a gas retaining chamber into which a gas to be measured (measuring gas) is introduced through an introduction orifice.
In this type of oxygen concentration detection device, the solid electrolyte member and the pair of electrodes together operate as an oxygen pump unit. When a drive current is supplied between the electrodes so that the electrode located on the gas chamber's side operates as a negative electrode, oxygen in the gas filling in the gas retaining chamber is ionized and migrates toward the surface of the electrode operative as a positive electrode. The oxygen ions are released through the surface of the positive electrode in the form of oxygen gas. The critical value of the current which can flow between the electrodes under this condition becomes substantially constant with regard to the change in the magnitude of supply voltage. On the other hand, it becomes proportional to the oxygen concentration in the measuring gas. Therefore, by detecting the magnitude of the critical current, the oxygen concentration in the measuring gas can be detected.
However, with the thus constructed oxygen concentration detection device, the output signal whose magnitude is proportional to the oxygen concentration is obtained only when the air/fuel ratio of the mixture detected in terms of the oxygen concentration in exhaust gas is on the lean side from the stoichiometric air/fuel ratio. Therefore, it was not possible to set a target air/fuel ratio value for the feedback air/fuel ratio control in a rich range of the air/fuel ratio.
Another example of an oxygen concentration detection device which can produce an output signal whose level is proportional to the oxygen concentration in exhaust gas both in the rich range and a lean range of the air/fuel ratio is described, for example, in Japanese Patent Application laid open No. 59-192955. It and which includes a pair of solid electrolyte members each of which is provided with a pair of electrodes. The surface of one of two electrodes provided on each solid electrolyte member respectively forms a part of the gas retaining chamber which in turn communicates with the measuring gas via an introduction orifice. The surface of the other electrode of one of the solid electrolyte members faces an atmospheric air chamber.
For this oxygen concentration detection device, one of the oxygen-ion conductive solid electrolyte members and two of its electrodes are operative as the sensor cell unit for sensing the oxygen concentration; the other one of the oxygen-ion conductive solid electrolyte members and its two electrodes are operative as an oxygen pump unit. With this construction, a drive current is supplied in such a manner that the oxygen-ions in the oxygen pump unit move toward its electrode located on the gas retaining chamber's side when a voltage generated across the electrodes of the oxygen concentration detecting sensor cell element is higher than a predetermined reference voltage. On the other hand, when the voltage across the electrodes of the sensor cell element is lower than the predetermined reference voltage, the drive current is supplied so that the oxygen-ions move toward the electrode which is located on the opposite side of the gas retaining chamber. In this way, the variation of the current value becomes proportional to the oxygen concentration both in the lean range and the rich range of the air/fuel ratio.
However, in this type of oxygen concentration detection device, it was necessary to form an atmospheric air chamber, Also, the configuration of the device was rather complicated and expensive.